A generator is one key piece of equipment of a power generation plant and allows converting mechanical energy to electrical energy. An AC generator comprises two primary components: a rotor and a stator. The rotor is made up of electrically conductive coils that form a rotor winding. The stator is also made up of coils that form a stator winding. Mechanical energy turns the rotor relative to the stator while a field current is passed through the rotor winding to induce a voltage on the stator winding. The amount of power produced by the generator is typically limited by the design characteristics of the generator. For example, one needs to consider heating effects that may develop due to increased levels of currents passing through the stator or the rotor windings. To reduce these heating effects, a suitable cooling fluid, such as hydrogen gas, is commonly used in large generators. Increasing the gas pressure, and thus the flow of the cooling fluid, may increase the stator and field current capability limits.
Generator manufacturers normally specify the power capability limits of a generator in a machine-specific capability curve, such as that shown in FIG. 1. For hydrogen-cooled machines, a family of static capability curves at various nominal hydrogen pressures up to rated pressure may be provided. Plant operators use these types of curves to monitor the output of the generator in terms of reactive power (e.g., megavars) and real power (e.g., megawatts) to ensure that the generator is operated within its capability limits, thereby preventing potentially harmful overheating of the generator and/or costly plant shutdown.
Operators often operate hydrogen-cooled machines at pressures below the rated design conditions to, for example, eliminate unnecessary and costly gas leakage. Depending on how far below the rated hydrogen pressure the generator is operated, there may be significant underutilization of the power-generating capability of the generator.